Although the object—piece of an old Russian Cosmos communications satellite—bypassed them, the danger posed by the growing collection of orbital debris is quite real. A good-sized area of Earth's orbital environment has become a virtual junkyard. Debris has been accumulating since the beginning of the Space Age in the late 1950s. Today, the amount of space debris is estimated to be in the tens of millions, ranging from spent rocket stages, old and non-functioning satellites, to tiny particles of rubble, the result of collisions or simple erosion of spacecraft that have been up in space, some of them for decades.

The incident last week isn’t the first time astronauts have been threatened by space junk; the ISS had two other close calls last year. Usually the space station performs what NASA calls a “debris avoidance maneuver”—simply moving itself slightly out of harm’s way. The ISS might be having more space junk encounters since changing position in the last year, moving into an area with a slightly higher density of orbital debris.

Protecting the space station from debris is a problem that requires vigilance from ISS support teams on Earth. It’s also a problem that isn’t going away any time soon.

NASA’s been concerned about the issue for a while. After Donald Kessler predicted in 1978 that over time, the density of space junk would grow to a point of making collisions inevitable—and Earth orbit unusable, in 1979, the space agency opened the Orbital Debris Program Office at the Johnson Space Center in Houston, Texas—and put him in charge of it.

Gene Stansbery, the office’s program manager, says even the smallest fragments of orbital debris can be harmful to spacecraft in orbit around the Earth—such as the ISS and communications and weather satellites we depend on. Even the tiniest pieces of space trash are dangerous, in part because they’re moving at tremendous speeds, at a velocity of 11 kilometers per second. Early in the space shuttle program, during the STS-7 mission, a tiny paint fleck hurtling through space hit a shuttle window, causing so much damage the entire window had to be replaced.

NASA’s Orbital Debris Program office isn’t the only US agency keeping an eye on wayward space junk. The US Department of Defense, according to Stansbery, has a worldwide network that can track objects, in low Earth orbit, as small as 10 centimeters in size.